Topotecan and Ginkgolic Acid Inhibit the Expression and Transport Activity of Human Organic Anion Transporter 3 by Suppressing SUMOylation of the Transporter.

Organic anion transporter 3 (OAT3), expressed at the basolateral membrane of kidney proximal tubule cells, facilitates the elimination of numerous metabolites, environmental toxins, and clinically important drugs. An earlier investigation from our laboratory revealed that OAT3 expression and transport activity can be upregulated by SUMOylation, a post-translational modification that covalently conjugates SUMO molecules to substrate proteins. Topotecan is a semi-synthetic derivative of the herbal extract camptothecin, approved by the FDA to treat several types of cancer. Ginkgolic acid (GA) is one of the major components in the extract of leaves that has long been used in food supplements for preventing dementia, high blood pressure, and supporting stroke recovery. Both topotecan and GA have been shown to affect protein SUMOylation. In the current study, we tested our hypothesis that topotecan and GA may regulate OAT3 SUMOylation, expression, and transport function. Our data show that the treatment of OAT3-expressing cells with topotecan or GA significantly decreases the SUMOylation of OAT3 by 50% and 75%, respectively. The same treatment also led to substantial reductions in OAT3 expression and the OAT3-mediated transport of estrone sulfate, a prototypical substrate. Such reductions in cell surface expression of OAT3 correlated well with an increased rate of OAT3 degradation. Mechanistically, we discovered that topotecan enhanced the association between OAT3 and the SUMO-specific protease SENP2, a deSUMOylation enzyme, which contributed to the significant decrease in OAT3 SUMOylation. In conclusion, this study unveiled a novel role of topotecan and GA in inhibiting OAT3 expression and transport activity and accelerating OAT3 degradation by suppressing OAT3 SUMOylation. During comorbidity therapies, the use of topotecan or extract could potentially decrease the transport activity of OAT3 in the kidneys, which will in turn affect the therapeutic efficacy and toxicity of many other drugs that are substrates for the transporter.